SYSTEM FOR SCANNING A TRANSMITTED BEAM THROUGH A 360º FIELD-OF-VIEW

ABSTRACT

A light detection and ranging (LIDAR) system including a tunable laser beam source that generates a modulated laser beam over a frequency modulation range; a spiral phase plate resonator (SPPR) device responsive to the modulated laser beam and providing a transmitted beam; and a mirror responsive to the transmitted beam and directing the transmitted beam at a certain angle therefrom depending on the frequency of the laser beam.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 16/418,654, titled, Frequency Modulated Scanning LIDAR With 360Degrees Field-Of-View, filed May 21, 2019.

BACKGROUND Field

The present disclosure relates generally to a LIDAR system, and moreparticularly, to a LIDAR system that employs a tunable laser source thatgenerates a laser beam and a spiral phase plate resonator (SPPR) devicethat receives and confines the beam to a narrow field-of-view (FOV),where the frequency of the laser beam is tuned to scan the narrow FOVlaser around a 360° FOV with no moving parts.

Discussion

Light detection and ranging (LIDAR) is a process that transmitsmodulated pulsed optical beams that are reflected off of a target andthe return beam is detected, where the time of flight of the beam isused to determine the distance to the target. By providing a point cloudof distance measurements in this manner an image of the object can beconstructed. A LIDAR system that performs such range finding typicallyincludes a laser beam source, an element to provide scanning of thelight beam, and detector(s) for detecting beam reflections.

LIDAR systems are used in many applications, such as mapping terrainusing aerospace vehicles, self-driving or autonomous vehicles, mappingstationary objects, constructing 3D models of rooms and other objectsfor use in a variety of computer applications including, for example,games, social media, communications applications, etc. For manyapplications there is an interest in scanning the transmitted beam abouta field-of-view (FOV) of a full 0 to 360°, such as for autonomousunmanned aerospace vehicles, manned aerospace vehicles, and self-drivingvehicles.

To scan a light beam in a LIDAR system, movable mechanical componentsare typically employed. In many cases, the light beam scanning componentis a mirror that is rotated to direct a fixed beam of light towards themirror, or the light beam source is mounted on a gimbal and the gimbalis rotated. In one design, a mirror is mounted to an electro-mechanicalcomponent, such as a piezoelectric element, and is controlled to changethe direction it faces. Regardless of the mechanical and/or electricalcomponent that provides the force to produce the change in the beamdirection, a physical change of position of the mirror or of the beamsource itself is employed. If there are mechanical movable parts, thesystem runs a higher risk of mechanical failure. Also, the speed ofrotation is limited by these mechanics as well as the usual wearconsiderations for a component expected to perform a very large numberof cycles over its lifetime. Alternately, multiple LIDAR systems couldbe combined to cover the full 0 to 360° FOV.

There is an increasing demand for LIDAR systems that have no movableparts and provide an instantaneous FOV over the full range of azimuthalangles from 0 to 360° at low cost. This demand is generally driven bythe desire to produce an imaging and ranging system for manned andunmanned autonomous land, air and sea vehicles, including guidancesystems for self-driving vehicles as well as systems for terrain mappingand structure mapping. However, it is challenging to achieve a full 0 to360° projected scanning range in a single LIDAR system without usingmovable parts.

Micro-electrical mechanical system (MEMS) devices have been employed toscan the beam in a LIDAR system. While the amplitude of oscillations ofMEMS devices are small and the frequency of the scanning is high suchthat there is lower risk for mechanical failure, the scanning FOV istypically limited. Optical phased array technology is an emergingtechnology, but the scanning FOV is also limited and ranging distance isrelatively short. Liquid crystal scanners are known to be used for thispurpose, but require active electrical control of the liquid crystal andthey have small angular range. There have been fundamental scientificstudies showing a resonator effect for a low reflectivity SPPR device.These systems offer a number of advantages and disadvantages forproviding a 360° FOV. However, there still exists a need for an opticalscanner that can scan a light beam about a substantial FOV without usingmovable components, especially if operated at high scan rates.

U.S. patent application Ser. No. 15/928,347, filed Mar. 22, 2018,titled, Scanning An Optical Beam About A Field of Regard With No MovingParts, assigned to the assignee of this disclosure and hereinincorporated by reference, discloses employing a spiral phase plateresonator (SPPR) as a 0 to 360° scanning mechanism with no movable partsfor a LIDAR system. An SPPR device is a miniaturized optical element forwhich a coherent superposition of optical vortices can be generated byeither reflecting light off of the device or transmitting light throughthe device. Because the SPPR device is based on the interference ofoptical vortices in a single SPPR it has many advantages compared toother popular devices that generate optical vortices, including thecompactness of the SPPR device, high power tolerance, the ability togenerate optical vortices in the reflective or transmissionconfiguration, as well as resilience to vibrations and misalignment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a LIDAR system including an SPPR devicefor providing a full 360° FOV;

FIG. 2 is a front view of a transmitter sub-system in the LIDAR system;

FIG. 3 is a top view of the transmitter sub-system;

FIG. 4 is a cross-sectional top view of a receiver sub-system in theLIDAR system;

FIG. 5 is a schematic block diagram of the LIDAR system showing thetransmitter sub-system that transmits an amplitude modulated scannedlaser beam and the receiver sub-system that receives and processes usingdirect detection a reflected beam from a target;

FIG. 6 is an illustration of the transmitted laser beam on the outputplane of the SPPR device in the transmitter sub-system;

FIG. 7 is an illustration of a reflected laser beam on the input planeof the SPPR device;

FIG. 8 is an illustration showing a horizontal c and vertical dfield-of-view of the transmitted laser beam;

FIG. 9 is a block diagram of a processor sub-system in the LIDAR systemincluding a central signal processor and secondary signal processorsinterfacing with the transmitter and receiver sub-systems;

FIG. 10 is a schematic diagram of a circuit showing signal processingelements for detectors in the transmitter sub-system;

FIG. 11 is an illustration of a detector in the transmission sub-systemthat determines the orientation of the scanned beam;

FIG. 12 is a schematic diagram of a circuit showing signal processingelements for the signal generated by the detector shown in FIG. 11 ;

FIG. 13 is a schematic diagram of a circuit for controlling thefrequency of a DFB laser beam source in the transmitter sub-system;

FIG. 14 is a schematic diagram of a circuit for controlling a driverthat controls a modulator for modulating the laser beam in thetransmitter sub-system;

FIG. 15 is a block diagram of a circuit that reads the temperature ofthe transmitter sub-system and the temperature of the receiversub-system;

FIG. 16 is a schematic diagram of a circuit for processing the reflectedbeam from the target in a receiver module;

FIG. 17 is a schematic diagram of a circuit for providing a bias voltageinput to a single pixel detector;

FIG. 18 is a block diagram of a display system for displaying a pointcloud of range data;

FIG. 19 is a flow chart diagram showing an operation of the LIDAR systemdepicted in FIG. 5 ;

FIG. 20 is a flow chart diagram showing a process for how the algorithmobtains the temperatures of the transmitter sub-system and the receiversub-system;

FIG. 21 is a flow chart diagram describing internal tracking and lockingof beam orientation to wavelength in the LIDAR system depicted in FIGS.5 and 27 ;

FIG. 22 is a flow chart diagram describing switching between differentsingle pixel detectors;

FIG. 23 is a graph with laser beam frequency sweep on the vertical axisand time on the horizontal axis showing frequency modulation of atransmitted beam in the LIDAR system depicted in FIG. 5 and a reflectedbeam off of a target;

FIG. 24 is a graph with beat frequency on the vertical axis and time onthe horizontal axis showing the optical beat frequency between thetransmitted optical beam and the received optical beam vs timerelationship due to the frequency modulated transmitted beam;

FIG. 25 is a graph with laser beam frequency on the vertical axis andtime on the horizontal axis showing frequency modulation of atransmitted beam in the LIDAR system depicted in FIGS. 5 and 27 and areflected beam off of the target over a stepped increasing scan infrequency of the transmitted beam for 360° scanning;

FIG. 26 is an expanded portion of the graph shown in FIG. 25 ;

FIG. 27 is a schematic block diagram of a LIDAR system including atransmitter sub-system that transmits a frequency modulated scannedlaser beam and a receiver sub-system that receives and processes areflected beam from a target using heterodyne detection or homodynecoherent detection; and

FIG. 28 is a flow chart diagram showing an operation of the LIDAR systemdepicted in FIG. 27 .

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the disclosure directedto a LIDAR system that employs a tunable laser source that generates alaser beam and a spiral phase plate resonator (SPPR) device thatreceives and confines the beam to a narrow field-of-view (FOV) is merelyexemplary in nature and is in no way intended to limit the disclosure orits applications or uses.

As will be discussed in detail below, the present disclosure describes aLIDAR system including a transmitter sub-system, a receiver sub-system,and a signal processor sub-system. The transmitter and receiversub-systems are synchronized to provide feedback to each other viasignal processors. The signal processor sub-system also computes therelevant quantities and outputs a point cloud of ranging distances.

FIG. 1 is an isometric type diagram of a LIDAR system 10 that includes atransmitter sub-system 12 and a receiver sub-system 14. FIG. 2 is afront view and FIG. 3 is a top view of the transmitter sub-system 12 andFIG. 4 is a top, cross-sectional type view of the receiver sub-system14. The transmitter sub-system 12 includes a narrow linewidth coherentlaser beam source 20, such as a distributed feedback (DFB) laser, wherethe source 20 can be frequency tuned and operate in a rugged environmentwith limited mode hopping, and that emits a coherent laser beam 18, forexample, in the visible-IR frequency range to an opto-electronicarchitecture 16 that includes various optical components depending onthe particular application. For example, the architecture 16 may includean optical isolator to prevent back reflection of the laser beam 18 intothe laser beam source 20. Specifically, light reflected back into thelaser beam source 20 may cause beam jitter in the laser cavity causingintensity fluctuations of the output laser beam 18 or spurious frequencyshifts in the laser wavelength, which could cause instability of themodes in the laser cavity and cause the laser beam source 20 to go outof lock. The beam 18 from the optical isolator is fiber coupled into amodulator, for example, an acousto-optic modulator (AOM), that modulatesthe laser beam intensity and a drive circuitry keeps track of the timingof the laser pulse modulation. The architecture 16 may also include awaveguide, such as an optical fiber, that maintains the beam 18 in asingle optical mode, for example, the TEM₀₀ Gaussian mode. In analternate embodiment, other optical elements instead of the single modeoptical fiber can be employed to provide the TEM₀₀ Gaussian mode. Thearchitecture 16 may also include a polarizing beam splitter (PBS) thatseparates a light beam based on its polarization, discussed in furtherdetail below, and a polarization rotator (PR). Although a DFB laser isthe laser beam source 20 in this embodiment, a plurality of other narrowlinewidth or pulsed laser sources are suitable such as frequency tunablediode lasers, frequency tunable fiber lasers, external cavity lasers,and distributed Bragg reflector (DBR) lasers. Depending on the output ofthe laser beam source 20, and possibly the power rating of the otherelectro-optical components, an optical amplifier could be includedbefore the PBS for high output power.

The single mode beam 18 is sent through an etalon 22 and into an SPPRdevice 26 that modifies the shape of the beam 18 and emits it towards aconical reflector 24 to be scanned in a 360° FOV, where the scandirection is based on an increasing or decreasing optical frequency ofthe laser beam 18. Speckle at the target can be avoided by using adiffuser around the conical reflector 24 or engineering rough surfaceson the conical reflector 24. The '347 application referenced aboveprovides a more detailed discussion of the SPPR device 26 in thiscontext. The SPPR device 26 includes an optically transparent block 28,such as glass, and a step-wise spiral reflector 32, such as a polymerlayer having a reflective material coating, with an azimuthally varyingstep 36 having height Δh on an output side of the block 28 that is alsooptically transmissive enough so that an output beam can propagatetherethrough and be output from the device 26. Although smoothreflective material coatings are employed in this design to providereflective surfaces on the block 28, in alternate designs nanoscalestructures can be used to provide the reflectivity of the beam 18 in theblock 28.

If the beam 18 propagated through the block 28 with no surfacereflectivity, an optical vortex beam with a well-defined winding numberwould be produced on the output plane of the device 26, where the device26 would act as a spiral phase plate. By providing finite reflectivityon opposing surfaces of the device 26 and providing the reflector 32having the gradually varying azimuthal thickness, the device 26 operatesas a spiral phase plate resonator, where the beam 18 is output from thedevice 26 as a coherent superposition of optical vortices separated byspecific positive winding numbers. In other words, each reflection ofthe beam 18 within the device 26 creates an individual optical vortexbeam having a unique phase that is output from the device 26.

The receiver sub-system 14 includes a cylindrical housing 40 that housesthe various processors and electrical hardware associated with thesystem 10 discussed below. A number of detector modules 42 eachincluding a fast and sensitive detector 44, such as avalanchephotodiodes (APD) or silicon photomultiplier (SiPM), are mounted to thehousing 40 in a certain configuration so that the detector 44 is on anouter surface 46 of the housing 40, such as in suitable rows andcolumns, to be in a position to detect the reflected beam off of thetarget of interest in any beam direction. In a different embodiment,single photon avalanche photodiodes (SPAD) could also be used. Eachdetector module 42 includes a circuit board 48 that extends into thehousing 40. While a network of thirty-six single pixel detectors areprovided to cover the full 360° FOV in this embodiment, where thecolumns of the detectors 44 are 30° apart on the housing 40, there couldbe a different number of detectors in a plurality of arrangements forthe detector's field-of-view.

FIG. 5 is a schematic block diagram of a LIDAR system 50 that representsone non-limiting example of the LIDAR system 10, where the system 50includes a transmitter sub-system 52, a receiver sub-system 54 includinga plurality of detector modules 56 and a signal processor sub-system 58.The transmitter sub-system 52 includes a laser beam source 70, anisolator 72, a modulator 74, a waveguide 76, a PBS 78, a PR 80, an SPPRdevice 82 mounted to a high finesse etalon 68 and a conical mirror 84that provides a 360° FOV transmitted and scanned beam 86 in the mannerdiscussed above that is reflected off of an object 88 as reflected beam108 to be received by the receiver sub-system 54 so that the distance tothe object 88 can be determined. It is noted that although notspecifically shown the laser beam 86 generated by the laser beam source70 would likely be amplified by an optical amplifier. The forwardpropagating laser beam at every stage through the transmitter sub-system52 and being transmitted will be referred to as the beam 86 below.

The beam 86 from the modulator 74 is fiber coupled into the waveguide 76by a collimator (not shown) at the end of the waveguide 76, and into thePBS 78. The PR 80 rotates the polarization of the beam 86 by 45° duringeach passage of the beam 86 into the device 82. More specifically, for alight beam that goes through the PR 80 in the forward direction thenback through the PR 80 in the backward direction, the polarization isrotated by 90°. The beam 86 is then coupled into the SPPR device 82,where a backward propagating beam 122 due to a weak reflection from theSPPR device 82 is created, for which the orientation of the beam 86 isinternally determined. The combination of the PR 80 and the PBS 78reduces the backward propagating beam 122 going to the laser beam source70. The reflected beam 122 from the SPPR device 82 is polarizationrotated by the PR 80 to have a different polarization than the incomingbeam 86 so that it is reflected by the PBS 78 instead of beingtransmitted through it. The transmitted beam 86 emerging from the SPPRdevice 82 is collimated and the beam 86 is reflected off the conicalmirror 84 into the desired FOV.

FIG. 6 is an illustration of the transmitted laser beam 86 at the outputplane of the SPPR device 82 showing a light area 62 in the direction ofthe beam 86, FIG. 7 is an illustration of the reflected laser beam 122at the input plane of the SPPR device 82 showing a null 64, and FIG. 8is an illustration of the beam 86 showing a horizontal c and vertical dfield-of-view.

Scanning of the beam 86 is achieved by the SPPR device 82 in thetransmitter sub-system 52 according to:

$\begin{matrix}{{\phi_{0} = {2\pi\frac{h_{0}}{\Delta h}\frac{1}{v}\delta v}},} & (1)\end{matrix}$

where ϕ₀ is the angular position of the optical beam emerging from thetransmitter, δv is the change in laser frequency, v is the center laserfrequency, Δh is the azimuthal step height of the device 82, and h₀ isthe substrate height. Equation (1) assumes that Δh «h₀, which is typicalfor most SPPR designs, and the device 82 has uniform refractive index.

The rotation rate of the beam 86 is defined as:

$\begin{matrix}{\frac{d\phi_{0}}{dt} = {2\pi\frac{h_{0}}{\Delta h}\frac{1}{v}{\frac{\delta v}{dt}.}}} & (2)\end{matrix}$

From the above equations, it is clear that the scan rate of the laserbeam 86 is directly proportional to the rate of change of the frequencyof the laser beam. Because the laser beam source 70 can be tuned quitefast, the tuning rate is exemplified in the 360° scanning of the beam86.

If the beam 86 is broadened to cover the entire tuning range of the SPPRdevice 82 without the etalon 68, the beam 86 emerges from the conicalmirror 84 in the entire 360° FOV simultaneously, thus degrading the scanresolution. The etalon 68 serves as an optical filter if a tunablebroadband laser is used, nevertheless, the increments in laser beamfrequency tuning allows for changing the orientation of the beam 86within a 360° angle. Alignment is maintained through the opticalcomponents in the transmitter sub-system 52. This is because most of theoptical elements are fiber coupled to each other with a small air gapbetween the optical elements from the PBS 78 to the SPPR device 82. Thisallows for preserving the back reflected optical mode that givesinformation on the orientation of the beam 86. The air gap is largeenough to allow for decay of the evanescent field and reject internalscattered light, but small enough to keep the system compact. Theoptical elements surrounding this air gap are rigidly held in place toavoid movement. This alignment is expected to be maintained even in thepresence of system vibrations.

The signal processor sub-system 58 coordinates and synthesizes thedigital signals going into the transmitter and receiver sub-systems 52and 54. This includes the timing of the pulses, modulation frequency,the magnitude of the beam 86, and phases of the electronic signals forthe transmitter and receiver sub-systems 52 and 54. It also provides aplatform for heterodyning transmitter and receiver electronic digitalsignals. In other words, the transmitter and the receiver signals thatare obtained by direct detection are electronically mixed to determinethe time delay of the amplitudes of the signals. The sub-system 58 mayinclude a flexible digital signal processor (DSP) having a fieldprogrammable gate array (FPGA), which can generate, receive andsynthesize signals. The FPGA digital processors have built-in finitepulse response filters (and other filters), fast Fourier functions,discrete cosine functions, as well as built-in addition, subtraction andaccumulation units to efficiently combine multiplications results. Theprocessors also have multiple input/output interfaces to communicatewith the transmitter and receiver sub-systems 52 and 54, and interfacesfor external memory, and other controls. Multiple FPGAs can be connectedin parallel for fast processing. In a different embodiment,microcontroller units or graphical processing units (GPU) can be used asthe digital signal processors, or parts of the digital signalprocessors.

There are several secondary signal processors which interface with thecomponents in the transmitter and receiver sub-systems 52 and 54. Thesecondary processors are connected with a central signal processor thatcomputes the signal outputs using a number of algorithms. FIG. 9 is ablock diagram of the signal processor sub-system 58 that includes acentral signal processor 90, a secondary signal processor 92 thatcontrols the laser beam source 70 and the modulator 74, a secondarysignal processor 94 that receives and processes detector signalsdiscussed in detail below, a secondary signal processor 96 that receivestemperature signals from transmitter sub-system and receiver sub-systemtemperature sensors also discussed in detail below and a secondarysignal processor 98 that controls the receiver sub-system 54. Thevarious processor clocks are also synchronized for appropriate timing.There is also an input to the central signal processor 90 to rundiagnostics on the system 50.

Select algorithms implemented by the central signal processor 90 inconjunction with the secondary signal processors 92, 94 and 96 includean algorithm to scan the transmitted beam from 0 to 360° and detect thereflected beam 108 by the receiver sub-system 54, an algorithm forinternal tracking of the beam position, an algorithm for switchingbetween different single pixel detectors, and an algorithm for reportingthe temperature of the transmitter and receiver sub-system 52 and 54.The exchange between the signal processor 92 and the central signalprocessor 90 controls the scanning of the frequency of the laser beam86, as well as the timing of the modulator 74. The exchange between thesecondary signal processor 94 and the central signal processor 90controls the integration times for the detectors, discussed below, inthe transmitter sub-system 52, as well as the determination of theinitial orientation of the transmitted beam 86, and periodic calibrationand locking of the beam orientation to the wavelength of the beam 86.The exchange between the central signal processor 90 and the secondarysignal processor 96 controls the detectors in the detector modules 56,including the bias voltage for switching between the detector modules56, as well as timing and gain of the reflected beam 108. There could bevariable integration times for the detector modules 56. The exchangebetween the secondary signal processor 98 and the central signalprocessor 90 read the chip temperature of the transmitter and receiversub-systems 52 and 54, and correct any temperature induced changes. Itis noted that the signal processors 92, 94, 96 and 98 exchangeinformation through the central signal processor 90. A more detaileddiscussion of these algorithms is given later in this discussion.

The reflected beam 122 from the PBS 78 is split by a beam splitter 100,where one of the split beams is sent to a detector 102 and the othersplit beam is sent to a detector assembly 104 including a number ofdetectors 106 in the transmitter sub-system 52. The detector 102 is afast detector, such as a fast diode, that keeps track of the timing andthe intensity modulation of the beam 86 in the SPPR device 82, as wellas any shifts in the longitudinal modes through a change in freespectral range of the etalon 68 that may occur. The electrical signalsfrom the detector 102 are conditioned by a circuit 110 before being sentto the secondary signal processor 94.

FIG. 10 is a schematic diagram of the circuit 110 that processes theelectrical signals from the detector 102, where the detector 102 isillustrated as a diode 112 and the secondary signal processor 94 isillustrated as a signal processor 114. The electrical signal from thediode 112 is amplified by a transimpedance amplifier (TIA) 116, bandpassfiltered by a band-pass filter 118 and then converted to a digitalsignal by an analog-to-digital converter (ADC) 120. The electricalsignal from the detector 102 modifies the signal provided to the laserbeam source 70 and select parts of the receiver sub-system 54. Some ofthe electrical signals from the transmitter sub-system 52 are mixed withthe electronic signal from the appropriate detector modules 56 in thereceiver sub-system 54 for distance range determination, and errorcorrection.

The angle orientation of the beam 86 as it is reflected off of theconical mirror 84 into the FOV is internally measured by the detectorassembly 104. More specifically, the orientation angle is measuredinternally by measuring a weak back reflection from the SPPR device 82.The detector assembly 104 forms the internal mechanism for recognizingthe beam's azimuthal angle in real time, as well as for calibrating andlocking the wavelength to the angular position of the transmitted beam86. The feedback provided to the secondary signal processor 94 alsoallows for appropriate power cycling among the detector modules 56 inthe receiver sub-system 54.

FIG. 11 is a schematic diagram of the detector assembly 104 showing thebeam splitter 100 splitting the reflected beam 122. The reflected beam122 is then projected into the diffraction far field using a short focallength lens 130, such as a GRIN lens. An aperture device 132 is placedbehind the lens 130 and includes eight apertures 134 that areindividually fiber coupled into eight single pixel detectors 136, suchas fast diodes. As the frequency of the laser beam 86 is tuned from onefrequency to another, light will not go through one or more of theapertures 134. Since each aperture 134 is fiber-coupled into itscorresponding detectors 136, the detector 136 that reads the null 64 isindicative of the beam's azimuthal angle, and thus the detector 136 alsoreads the corresponding scanning projection angle of the beam 86.

Although the detector assembly 104 includes eight detectors 136separated by 45°, there could be more or less single pixel detector andaperture combinations. To improve accuracy and precision, theaperture-detector combination could be arranged along a circle in anasymmetric fashion. That is, some aperture-detector combinations areseparated by 45° while others are separated by smaller or larger angleswith more significant figures. That way, once these angles are known,the exact initial orientation of the beam is also known.

The electrical signals from the detectors 136 are provided to a circuit140 to be processed before being sent to the secondary signal processor94. FIG. 12 is a schematic diagram of the circuit 140 showing thespecific elements and including a detector module 144 for each of thedetectors 136 and a signal processor 146 representing the secondarysignal processor 94. Each detector module 144 includes a transimpedanceamplifier 148 that receives and amplifies the signal from the detectormodule 144, a bandpass filter 150 that filters the amplified signal andan analog-to-digital converter (ADC) 152 that converts the amplified andfiltered signal to a digital signal. The digital signals from all of thedetector modules 144 are sent to the signal processor 146 so that thesystem 50 knows the orientation of the beam 86 as the wavelength of thelaser beam 86 from the laser beam source 70 is changed.

The secondary signal processor 92 sends a signal to the laser beamsource 70 to tune the frequency of the beam 86 through a circuit 160.FIG. 13 is a schematic diagram of the circuit 160 and showing a signalprocessor 162 representing the secondary signal processor 92 and a diode164 representing the laser beam source 70. The digital tuning signalfrom the signal processor 162 is converted to an analog signal by a DAC166 and is then amplified by a transconductance amplifier 168 thatcontrols an FET switch 170 to provide power to the diode 164. At aspecified start time, the DFB laser wavelength is tuned by changing thediode current to the diode 164, or via a suitable command signal.

The modulator 74 provides amplitude or intensity modulation of the laserbeam 86. The signal that drives the modulator 74 is also generated bythe secondary signal processor 92 and is provided to the modulator 74through a circuit 174 and a driver 176, where the circuit 170 wouldinclude a DAC for converting the digital signal from the processor 92 toan analog signal. It is noted that the laser beam 86 could be directlyintensity modulated by an AOM or a Mach-Zehnder modulator could beemployed to modulate the intensity with even higher bandwidths. Similarto an AOM, a bias would be applied to the Mach-Zehnder modulator.

FIG. 14 is a schematic diagram of a circuit 180 that could be used inplace of the circuit 174 and includes an amplitude modulator 182 thatreceives the digital signal from the processor 92, a mixer 184 thatreceives the amplitude modulation signal, and a radio frequencyamplifier 186 that amplifies the modulation signal.

A temperature sensor 190 measures the temperature of the transmittersub-system 52 and a temperature sensor 192 in each detector module 56measures the temperature of the receiver sub-system 54. The temperaturemeasurements are provided to a circuit 194 shown generally in FIG. 15that includes a clock generator 196. Periodic updates of the temperaturefrom the transmitter sub-system 52 adjust the wavelength of the laserbeam 86 to compensate for any slow thermal drifts experienced by theSPPR device 82 and the laser beam source 70. The back reflection fromthe etalon 68 also provides similar information through shifts in thelongitudinal modes, i.e., a change in the free spectral range of theSPPR device 82, on a very slow time scale compared to the fast timescales of the intensity modulations for distance ranging. Withspecialized algorithms, the thermo-optic effects in the SPPR device 82are not expected to result in deleterious measurements. This is becausethe time scales that the laser beam frequency is changing to rotate thebeam 86 is much faster than the time scales for which there aretemperature induced expansion of the SPPR device 82. One algorithm forreading the temperature to correct for any possible thermally inducedlaser frequency shifts and expansion of the SPPR device 82 is discussedlater in this description.

The beam 86 from the transmitter sub-system 52 travels to the object 88,and is reflected back as the reflected beam 108 into the receiversub-system 54 where it is detected by one or more of the detectormodules 56. A narrow band-pass optical filter 200 is placed in front ofeach of the modules 56 to significantly reduce any surroundingbackground light, e.g. sunlight. The frequency of the laser beam 86 istuned over a relatively narrow range Δv compared to the center laserfrequency v₀ to change the direction of the transmitted beam 86. Thiscan be quantified by the fractional change in optical frequency Δv/v₀going into the SPPR device 82. The narrow frequency range emerging fromthe transmitter sub-system 52 allows for the band-pass filter 200 to bemuch narrower than 1 nm, thus rejecting significant background light. Alens 202 is placed in front of a single pixel detector 204 in eachmodule 56 and will constrain the detector's FOV, thus further reducingbackground light, e.g. sunlight. In an alternate embodiment, the lightfrom the lens 202 is coupled into each detector 204 through an opticalfiber.

The electrical signals from the detectors 204 are sent to the secondarysignal processor 98 through a circuit 210 shown in FIG. 16 , where thesignal processor 98 is shown by a signal processor 214 and the detector204 is shown by diode 212. Particularly, the signal from the diode 212is amplified by a transimpedance amplifier (TIA) 216 and a variable gainamplifier (VGA) 218, where the VGA 218 receives a gain signal from thesignal processor 214, filtered by a band-pass filter 226 and thenconverted to a digital signal by an ADC 222.

The secondary signal processor 98 determines the appropriate gain levelfor the detector 204 through the VGA 218, and the appropriate modulationbias of the detector 204 through a circuit 224 to turn the detector 204on or off. FIG. 17 is a schematic diagram of the circuit 224 showing amodulation bias signal from the signal processor 214 being converted toan analog signal by a DAC 226, and then amplified by a TCA 228 beforebeing sent to the diode 212. Turning the detectors 204 on and off, i.e.,power cycling, at appropriate times is critical for low power operation.In an alternate embodiment, time to digital converters (TDC) could beused to directly convert pulse arrival times to the digital output forthe signal processors.

The central signal processor 90 displays the range finding informationon a display 230. FIG. 18 is a schematic block diagram of a system 232showing how the range finding information can be displayed. The datafrom the signal processor 90, represented by a signal processor 234, issent to a Fast Fourier Transform (FFT) module 236 that converts the datato the frequency domain and then to a memory array 238 for storage. Thedigital data from the memory array 238 is then converted to analog databy a DAC 240, which is then amplified by a VGA 242 and displayed on adisplay 244.

For some 0 to 360° FOV LIDAR systems, there may be cross-talk betweendifferent transmitter emitters and receiver detectors, where lightemitted by one laser source is inadvertently received by a differentdetector. The 360° FOV LIDAR system based on an SPPR device in thetransmitter alleviates this issue as there is only one transmitter thatcan scan a laser beam in the full 360° FOV, and each angle of theoutward propagating beam is locked to a corresponding receiver angleFOV. Switching between the different receiver detectors 204 as the laserbeam is scanned prevents cross-talk, thus reducing erroneous values indistance range estimation.

In order to allow the system 50 to scan the beam 86 from 0 to 360° anddetect the reflected beam 108 from the object 88, the laser beam source70 generates the laser beam 86 at frequency v₁, which corresponds to anorientation of the beam 86 reflected off of the mirror 84 at angle ϕ₁,where v₁∝ϕ₁∈[0, 2π]. The frequency v₁ is the first frequency of thelaser beam 86 to obtain the beam orientation at angle ϕ₁. Aftertemperature compensation, the signal processor 92 sends a command signalto increase/decrease the diode current of the source 70 to change thefrequency of the laser beam 86 to frequency v₂. At frequency v₂ thelaser beam 86 is then amplitude or intensity modulated at modulationfrequency Ω₀ by the modulator 74 for time τ₂ with N modulated intensitypulses. The laser beam frequency v₂ corresponds to a well-defined beamangle ϕ₁ propagating from the transmitter sub-system 52. The clock inthe processor 92 sends signals to the transmitter optical components,i.e., the laser beam source 70 and the modulator 74, to generate Npulses or modulations. This electronic signal is also synchronized withthe processors 94 and 96, which send electronic signals to turn therelevant detector modules 56 on and off in the receiver sub-system 54.To ensure a high signal-to-noise ratio, including mitigation ofmultipath interference, the modulator 74 is then modulated at severaldifferent intensity modulation frequencies, i.e., Ω₁, Ω₂, Ω₃, . . . ,for the laser beam frequency v₂ and the laser beam 86 is synchronizedwith the detector modules 56 that are “on” in the receiver sub-system54, see FIG. 21 discussed later. The distance range is then computedfrom the cross-correlation of the electronic digital signal sent tomodulate the beam 86 in the transmitter sub-system 52, electronicdigital signals receiver signals from the receiver sub-system 54 that isdetermined from the returned optical intensity incident on the column ofdetector modules 56, and a reference signal that is internally generatedby the signal processor 92. In some cases, the reference signal is thesame as the modulated electronic transmitted signal. Thiscross-correlation is performed by the digital signal processor 92 totrack the phase between the transmitted signal and the received signal,and consequently calculate the range to the object 88. The above stepsare repeated for frequencies v₃, v₄, v₅ . . . v_(N) of the laser beam 86until the beam 86 traces out the full 0 to 360°, i.e. ϕ₃, ϕ₄, ϕ₅=2π rad.The point cloud for distance ranging to targets that reflect thetransmitted beam 86 is calculated and displayed. For the purpose ofimproving the lateral resolution, the laser beam frequency can bedithered at each angle while scanning the beam 86.

The azimuthal reflection angle of the beam 86 is internally tracked inthe transmitter sub-system 52. The back reflection of the beam 86 fromthe SPPR device 82 is indicative of the beam orientation on the outputplane of the SPPR device 82. The position of each fast detector 136corresponds to the angle ϕ₁ of the beam 86 emerging from the transmittersub-system 52. One of the fast detectors 136 reads the back reflectedbeam, i.e., a null signal, and reports the beam angular position to thesignal processor 94. A weighted cross-correlation between the digitalsignal from the detector optical signal and the digital signal thatcorresponds to a particular laser frequency (laser current) is performedby the processor 92, which locks wavelength to orientation on the outputplane of the SPPR device 82. This is a dynamic locking scheme of thebeam orientation to wavelength. For a good measurement, the position ofthe transmitted beam 86 emerging from the system 50 is reported. For abad measurement, the frequency of the laser beam 86 is tuned inincrements and the steps are performed again until a good measurement isobtained. The time to perform this operation should be shorter than thetime it takes the beam 86 to make a full 360° scan. After initialcalibration measurement, this measurement is performed at successivetime intervals within the scanning of the beam 86 through the 360° FOV.This ensures precise and accurate angles of the beam 86 emerging fromthe transmitter sub-system 52, as well as appropriate power cycling ofthe detectors 204.

An algorithm switches different ones of the single pixel detectors 204in the receiver sub-system 54 to an on state and an off state dependingon the angular position of the beam 86 emitted from the transmittersub-system 52. For the specific frequency of the laser beam 86 thatcorresponds to the current or a command signal going into the laser beamsource 70, the processor 92 changes the present position of the laserbeam 86 to a new position of the laser beam 86. The bias voltage thatcorresponds to the new detector position is turned on, and the biasvoltage that corresponds to the old detector position is turned off. Ifa column of the detector modules 56 receives too much light from thereflected beam 108, then the algorithm integrates for a shorter periodof time and reduces the peak intensity of the beam 86 from thetransmitter sub-system 52. If the detector 204 receives too little lightfrom the reflected beam 108, the algorithm integrates for a longerperiod of time. The algorithm alternates between integrating for a shorttime and a long time for each scan angle. This enables more sources ofnoise to be rejected while ensuring fast operation speeds.

For reporting a change in temperature, the digital signal processor 96reads the initial temperature from the sensors 190 and 192. From thesemeasurements, the number and width of the longitudinal modes in the SPPRdevice 82 is estimated for which the free spectral range of the SPPRdevice 82 is calculated. For a change or shift in the longitudinal modesin the SPPR device 82, as well as a temperature change from the sensors190 and 192 in the sub-systems 52 and 54, respectively, the algorithmadjusts the frequency of the laser beam 86 in the transmitter sub-system52, and the bias voltage of the detector modules 56 in the receiversub-system 54. The time interval for which the temperature is read isdetermined from the transmitter scan speed.

The discussion above is illustrated by the following flow chartdiagrams. FIG. 19 is a flow chart diagram 250 showing the operation ofthe LIDAR system 50 as discussed above. At box 252, the algorithm setsthe optical frequency v₁ of the laser beam 86 generated by the laserbeam source 70 based on the temperature of some of the individualcritical components of the transmitter sub-system 52 and the receiversub-system 54 provided at box 254. The purpose of determining thetemperature is to ensure that any change in temperature that inducesthermo-optic effects in the optical and electro-optical components aswell as electronic components is adequately compensated. Most of theelectronic components already have built in temperature sensorsincluding the digital signal processors.

FIG. 20 is a flow chart diagram 260 showing a process for how thealgorithm obtains the temperatures of the transmitter sub-system 52 andthe receiver sub-system 54. The algorithm operating in the secondarysignal processor 96 reads the temperature measurements from the sensors190 and 192 at box 262 and the algorithm operating in the secondaryprocessor 94 reads the output of the detectors 102 and 104 at box 264.The algorithm then estimates the number and the width of thelongitudinal modes in the SPPR device 82 at box 266. From thismeasurement, the free spectral range of the SPPR device 82 can bedetermined. When there is a change or shift in the longitudinal modes inthe SPPR device 82 from the free spectral range, or the temperaturemeasurements from the sensors 190 and 192, the algorithm changes thefrequency of the laser beam 86 and the bias voltage of the detectormodules 56 in the receiver sub-system 54 at box 268. The algorithm thenreads the temperature at a rate determined by the scan speed of the beam86 at box 270.

Returning to FIG. 19 , the algorithm then shifts the optical frequencyof the beam 86 to frequency v₂ and then intensity modulates the beam 86at the intensity modulation frequency Ω₀ for time τ at box 272. Thealgorithm then internally tracks the intensity modulated beam 86 that istransmitted towards the object 88 at box 274.

FIG. 21 is a flowchart diagram 280 showing how the algorithm providesinternal tracking of the beam 86. The algorithm monitors the reflectedbeam 122 from the SPPR device 82 at box 282, puts the detectors 204 inthe on state at box 284 and reads the reflected beam 108 at one of thedetectors 204 at box 286. The algorithm then provides a weightedcorrelation of the detector digital signal and the signal correspondingto the laser beam frequency v₂ at box 288. The algorithm then reportsthe angular position of the laser beam 86 for a good measurement, andshifts to the frequency of the laser beam 86 and makes a measurementagain for a bad initial measurement at box 290. After an initialcalibration, the algorithm reports the measurements at successiveintervals at box 292.

Again returning to FIG. 19 , the algorithm then synchronizes the clocksin the processors 90, 92, 94, 96, 98, 114, 146, 162, 214 and 234 and theelectronic components in the opto-electronic components in thetransmitter sub-system 52 and the receiver sub-system 54 at box 295. Thealgorithm then systematically intensity modulates the laser beam 86 atmultiple intensity modulation frequencies Ω₁, Ω₂, Ω₃, . . . , Ω_(N) andsynchronizes the intensity modulated laser beam 86 with the detectormodules 56 at box 296. This synchronization occurs by the processors 90,92, 94, 96, 98, 114, 146, 162, 214 and 234 sending electronic signals tothe detectors 204 in the on state to receive the return opticalintensity pulses for which there are return electronic signals to theprocessor, and sending signals to the laser beam source 70 and themodulator 74. The algorithm then performs a correlation between theelectro-optic element signals, i.e., the laser beam source 70 and themodulator 74, in the transmitter sub-system 52, the detectors 204 in thereceiver sub-system 54 and reference signals generated by the signalprocessor 90 to compute range parameters to the object 88 at box 298.The processes from boxes 252, 272, 204, 296 and 298 is then repeated atbox 300 for different laser beam frequencies v₃, v₄, v₅ . . . v_(N) fora 360° FOV. This operation requires using different ones of the detectormodules 56, where switching between the modules 56 occurs at box 302.

FIG. 22 is a flow chart diagram 304 showing the process for switching orpower cycling between different ones of the detectors 204. At box 306,the algorithm reports the angular position of the beam obtained from theflow chart diagram 280. At each laser frequency, one or more columns ofthe detectors 204 are in the “on” state while the other detectors 204are in the “off” state. The power cycling between the “on” and “off”states in the network of the detectors 204 reduces power dissipation andconsequently reduces internal heating of the components. The spatialposition of the transmitted laser beam 86, which is proportional to thefrequency change of the laser beam 86, determines the column of thedetectors 204 that would be turned “on” from their “off” state duringthe angle scanning of the laser beam 86. This is done by synthesis ofthe internal signals from the detector assembly 104 in the transmittersub-system 52 for which the signal processor 98 returns a bias voltagethrough the circuit 224 to perform switching operations for the detectormodules 56. The response speed of these single pixel detectors exceedstens of megahertz, and thus power cycling between the detectors 204 isquite fast. The algorithm then turns on the proper column of thedetectors 204 at box 308 and receives the reflected beam 108 at box 310.During the scanning process, the algorithm turns on the bias voltagethat corresponds to the new detector position and the old detectorposition is turned off at box 312. The integration times are varied atbox 314 for different received light levels. The distance ranging pointcloud of the object 88 is then calculated and displayed at box 316.

The system 50 discussed above uses amplitude or intensity modulation ofthe laser beam 86 to extract the distance range point cloud of theobject 88. In an alternate embodiment, the system 50 can be modified toextract the distance range point cloud of the object 88 by usingfrequency modulation of the laser beam 86. Frequency modulating a laserbeam in a LIDAR system to obtain range and velocity of a target is knownin the art. However, the present disclosure combines that technique withthe SPPR device 82 to provide 360° scanning with no moving parts.Advantages of the frequency modulation method include unambiguouslyextracting distance and velocity information from the same opticalsignal, and most importantly long distance and high resolution ranging.In this embodiment, the modulator 74 does not amplitude modulate thelaser beam 86 relative to the travel time of the beam 86 to the object88 and back, but serves to add a frequency bias to the beam 86 bymodulating the intensity of the beam 86 with a long square pulse to gatethe transmitted laser beam 86 and the reflected beam 108 for noiserejection and timing synchronization.

In a LIDAR system of this type employing beam frequency modulation, thefrequency of the laser beam 86 is modulated by a linear triangularwaveform over a frequency range of ΔF=F₂−F₁, where F₁ and F₂ are thestart (lower) and the stop (higher) frequencies, respectively, of thewaveform for the beam 86. The time delay Δt from transmitting the laserbeam 86 to receiving the reflected beam 108 and the laser sweep rates_(laser), where:

$\begin{matrix}{{s_{laser} = \frac{F_{2} - F_{1}}{T}},} & (3)\end{matrix}$

are used to determine the distance range D_(R) to the object 88 from thefrequency f_(Dis), where:

$\begin{matrix}{{f_{Dis} = {{\Delta{ts}_{laser}} = {\frac{2D_{R}}{c}s_{laser}}}},} & (4)\end{matrix}$

and where C is the speed of light,

$T = \frac{1}{2f_{mod}}$

is the time or period to modulate the laser beam frequency with atriangular waveform, f_(Dis) is determined from intermediate laser beamfrequencies f_(IF1) and f_(IF2), and f_(mod) is the modulation frequencyof the waveform driving the laser beam 86 over a certain time period.

If the object 88 is stationary, there will be one intermediate frequencyf_(IF1), but if the object 88 is moving there will be two intermediatefrequencies f_(IF1) and f_(IF2), where the intermediate frequenciesf_(IF1) and f_(IF2) are determined from the beat frequencies ofrepresentative optical signals of the mixed transmitted beam 86 to theobject 88 and reflected beam 108 from the object 88. The distance to theobject 88 is determined from the average of the intermediate frequenciesf_(IF1) and f_(IF2) as:

$\begin{matrix}{f_{Dis} = {\frac{f_{{IF}1} + f_{{IF}2}}{2}.}} & (5)\end{matrix}$

If the object 88 is moving, the velocity of the object 88 is related tothe difference in the intermediate frequencies f_(IF1) and f_(IF2) bythe Doppler shift as:

$\begin{matrix}{{{f_{{IF}1} - f_{{IF}2}} = {{2f_{Doppler}} = {2\frac{Vv_{0}}{c}}}},} & (6)\end{matrix}$

where f_(Doppler) is the Doppler shifted frequency, V is the velocity ofthe object 88, and v₀ is the center frequency of the laser beam 86.

The relationship between the transmitted laser beam 86 that has beenfrequency modulated with a continuous or pulsed triangular waveform andthe resulting reflected beam 108 based on the discussion above isillustrated by the graph in FIG. 23 , where time is on the horizontalaxis and frequency is on the vertical axis. Modulation of the laser beam86 at a certain beam angle frequency v over a modulation frequency rangeΔF is shown by graph line 320 and the reflected beam 108 is shown bygraph line 322, where s_(laser) is the slope of the lines 320 and 322.FIG. 24 is a graph with time on the horizontal axis and beat frequencyon the vertical axis showing the beat frequencies in the beams 86 and108 over time. The intermediate frequencies f_(IF), and f_(IF2) aredetermined from mixing or correlating the reflected beam 108 from theobject 88 with the transmitted beam 86. The reflected beam 108 from theobject 88 can also be correlated with a local oscillator providing areference signal. The time delay Δt between the lines 320 and 322 andthe intermediate frequencies f_(IF1) and f_(IF2) are used to determinethe distance to the object 88 and the Doppler shift frequencyf_(Doppler) is used to determine the velocity of the object 88.

To provide the frequency modulation, the diode current that controlsfrequency of the beam 86 generated by the laser beam source 70 is tunedto change the narrow linewidth frequency of the laser beam 86, and hencerotate the transmitted laser beam 86 in the 360° FOV, where thecoherence length of the laser beam 86 should be at least twice theranging distance. Depending on the design parameters of the SPPR device82, the frequency tuning range Δv to scan the laser beam 86 over 360°could be anywhere from 10s of GHz to 100s of GHz. The signal processorsub-system 58 causes the laser beam source 70 to generate the laser beam86 so that it systematically increases in frequency linearly in astepped and ramped manner so that the laser beam 86 is projected intothe 360° FOV. The signal processor sub-system 58 also places the smalltriangular modulating waveform on the linearly increasing laser beamfrequency ramp to frequency modulate the laser beam 86 at each beamfrequency v, where the frequency range ΔF of this triangular waveformwill be on the order of 10s of MHz, and where the amount of modulationdoes significantly change the angle that the beam 86 is reflected off ofthe mirror 84. This allows for high lateral resolution and obtaining abeat frequency between the frequency of the transmitted laser beam 86and the frequency of the reflected beam 108 for distance measurement.

This beam frequency and modulation control is shown by the graphs inFIGS. 25 and 26 that illustrates as the frequency of the transmittedlaser beam 86 is stepped up in frequency over the frequency range Δv,the beam 86 is modulated with the triangular waveform. Graph line 324 isthe transmitted laser beam 86 and graph line 326 is the reflected beam108, where the graph of FIG. 25 is a blown up portion of the graph inFIG. 26 .

The signal processor sub-system 58 obtains the time delay Δt between thetransmitted beam 86 and the reflected beam 108. For the directdetection, the signal processor sub-system 58 also mixes the electronicsignal of the transmitted and received frequency domain chirped pulsesfrom the detectors 102, 106 and 204, and computes the fast Fouriertransform (FFT) of the signal to extract the intermediate frequenciesf_(IF1) and f_(IF2). The beat frequency of the modulated beam that isfrequency tuned to rotate the beam 86 over a 360 degree angle will havea similar shape. The distance to and the velocity of the object 88 canthen be computed.

The LIDAR system 50, whether it intensity modulates or frequencymodulates the beam 86, provides direct detection of the reflected beam108 in the receiver sub-system 54 as described. However, the receiversub-system 54 can be replaced with a balanced receiver that providescoherent detection. In other words, the transmitted beam 86 and thereflected beam 108 are electrically mixed for beam synchronization andcorrelation purposes in the system 50. However, the transmitted beam 86and the reflected beam 108 can also be optically mixed for the samepurpose.

FIG. 27 is a schematic block diagram of a LIDAR system 330 that providesfrequency modulation of the laser beam 86 in the manner discussed above,but where the transmitter sub-system 52 and the receiver sub-system 54have been modified to provide heterodyne or homodyne coherent detectionof the reflected beam 108 for optically mixing the transmitted beam 86and the reflected beam 108 instead of direct detection, and where likeelements to the system 50 are identified by the same reference number.In this embodiment, the split beam from the beam splitter 100 and thereflected beam 108 from the lens 202 are coherently or optically mixedin a 50-50 beam splitter 332, such as a 3 dB coupler, in the transmittersub-system 52 so that the transmitted beam 86 and the reflected beam 108are interfered with each other. The mixed beams are then sent to thedetector 102 in the transmitter sub-system 52 to provide the beamtracking and the detectors 204 in the receiver sub-system 54 to providethe return beam detection. The signals from the detectors 102 and 204are sent to the signal processor sub-system 58 that performs a FFT onthe mixed signals from the detectors 102 and 204 to convert the signalsto the frequency domain and generate beat frequencies as a function oftime. The sub-system 58 then estimates the intermediate frequenciesf_(IF1) and f_(IF2) from the beat frequencies, which are used todetermine the distance to and velocity of the object 88 in the mannerdiscussed herein. It is noted that in an alternate embodiment, the beam86 can be intensity modulated as described above and the beam 86 and thebeam 108 can be optically mixed using the beam splitter 332. Further,the techniques for internally track the rotation of the beam 86 (angleand rotation rate), and power cycling of the detectors 204 is largelythe same for both coherent detection techniques and the direct detectiontechniques of the reflected beam 108.

FIG. 28 is a flow chart diagram 340 showing the processes and algorithmsperformed in the signal processor sub-system 58 for determining thedistance to and the velocity of the object 88 using frequency modulationof the beam 86 for both the direct detection technique associated withthe system 50 and the coherent detection technique associated with thesystem 330. At box 342, the algorithm sets the frequency v₁ of the laserbeam 86 generated by the laser beam source 70 based on the temperatureof the individual critical components of the transmitter sub-system 52and the receiver sub-system 54, which provides a start angle ϕ of thebeam 86 off of the mirror 84. The frequency of the laser beam 86 isshifted to the next frequency step, which is frequency v₂, to adjust theangle ϕ of the beam 86 off of the mirror 84 at box 344. The beam 86 atthe frequency v₂ is modulated with a triangular waveform at themodulation frequency f_(mod) over the frequency range ΔF at box 346,where the range ΔF of the modulation frequency f_(mod) is ΔF<<Δv, whichprevents the beam 86 from walking off of the object 88 during distanceranging. For example, the system 50 or 330 can be designed so that therange ΔF of the modulation frequency f_(mod) is on the order of tens orhundreds of MHz, and the change in the frequency range Δv of the laserbeam 86 to provide the entire 360° FOV scan is tens of GHz or hundredsof GHz.

The frequency v₂ that the beam 86 is currently set to is subtracted fromthe detected reflected beam 108 at box 348, which leaves the modulationfrequency f_(mod). The algorithm then mixes and correlates thesubtracted reflected beam 108 from the object 88 with the transmittedbeam 86 at box 350. The mixing is performed in the optical domain forcoherent detection or in the electronic domain for direct detection, asdiscussed above. The algorithm calculates the FFT of the signalsrepresenting the mixed beams to convert the signals to the frequencydomain at box 352, determines the beat frequencies in the mixed andFourier transformed signals as a function of time at box 354, identifiesthe intermediate frequencies f_(IF1) and f_(IF2) from the beatfrequencies at box 356, estimates the time delay Δt between transmittedbeam 86 and the reflected beam 108 from the beat frequencies todetermine the distance to the object 88 at box 358 and determines theDoppler shift frequency f_(Doppler) from the beat frequencies todetermine the velocity of the object 88 at box 358. The algorithm thendetermines whether the 360° scan has been completed at decision diamond360, and if not, returns to the box 344 for a next step frequencyv₃-v_(n) to change the angle of the beam 86 off of the mirror 84 untilthe entire 360° FOV is swept. The steps at the boxes 346-358 arerepeated for each step frequency v, and once the 360° scan is completeat the decision diamond 360, the distance ranging point cloud andvelocity for the object 88 is calculated and displayed at box 362.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present disclosure. One skilled in the art willreadily recognize from such discussion and from the accompanyingdrawings and claims that various changes, modifications and variationscan be made therein without departing from the spirit and scope of thedisclosure as defined in the following claims.

What is claimed is:
 1. A light detection and ranging (LIDAR) systemcomprising: a tunable laser beam source that generates a modulated laserbeam over a frequency modulation range; a spiral phase plate resonator(SPPR) device responsive to the modulated laser beam and providing atransmitted beam; and a mirror responsive to the transmitted beam anddirecting the transmitted beam at a certain angle therefrom depending onthe frequency of the laser beam.
 2. The LIDAR system according to claim1 further comprising a modulator that modulates an intensity of thelaser beam.
 3. The LIDAR system according to claim 2 wherein themodulator is an acousto-optic modulator (AOM) or a Mach-Zhendermodulator.
 4. The LIDAR system according to claim 1 wherein the mirroris a conical mirror and the system determines a distance to and velocityof a target by scanning the transmitted beam through a 360°field-of-view (FOV) and processing beams reflected off of the target,and wherein the laser beam source tunes and shifts the optical frequencyof the laser beam to a plurality of frequencies to change the angle thatthe transmitted beam is directed from the conical mirror over a complete360° scan.
 5. The LIDAR system according to claim 4 further comprising amixer that is responsive to the transmitted beam from the SPPR deviceand the reflected beam from the target, said mixer optically mixing thebeams and providing one mixed beam to a timing detector and anothermixed beam to receiver detectors.
 6. The LIDAR system according to claim5 wherein the receiver detectors are part of a detector assembly thatincludes an aperture device having a plurality of apertures that areeach individually fiber coupled into a single pixel detector, andwherein as the frequency of the laser beam is tuned from one frequencyto another frequency, one of the single pixel detectors will read a nullthat is indicative of the angle orientation of the transmitted beam. 7.The LIDAR system according to claim 6 wherein the plurality of aperturesis eight apertures and wherein the single pixel detectors are configuredevery 45° around a circle.
 8. The LIDAR system according to claim 5wherein the mixer optically mixes the transmitted beam and the reflectedbeam through coherent detection.
 9. The LIDAR system according to claim5 wherein the mixer electrically mixes the transmitted beam and thereflected beam through coherent detection.
 10. The LIDAR systemaccording to claim 1 further comprising a first temperature sensor thatmeasures the temperature of a transmitter and provides first temperaturesignals and a second temperature sensor that measures the temperature ofa receiver and provides second temperature signals, wherein the firstand second temperature signals help tune the frequency of the laser beamgenerated by the laser beam source.
 11. The LIDAR system according toclaim 1 wherein the laser beam source frequency modulates the laser beamusing a continuous or pulsed triangular waveform.
 12. The LIDAR systemaccording to claim 11 wherein the laser beam source frequency modulatesthe laser beam by shifting the optical frequency of the laser beam in astepwise and ramped manner.
 13. The LIDAR system according to claim 1wherein the frequency modulation range is much less than the frequencyof the laser beam.
 14. The LIDAR system according to claim 13 whereinthe frequency modulation range is in the tens to hundreds of MHz and thefrequency of the laser beam is in the tens to hundreds of GHz.
 15. TheLIDAR system according to claim 1 further comprising a polarizing beamsplitter (PBS) that receives the laser beam before the SPPR device andreceives a reflected beam from the SPPR device, said PBS directing thereflected beam to a timing detector and a detector assembly based on itspolarization.
 16. The LIDAR system according to claim 15 furthercomprising a polarization rotator (PR) positioned between the PBS andthe SPPR device, said PR rotating the polarization of the reflected beamto have a different polarization than the laser beam entering the SPPRdevice.
 17. A light detection and ranging (LIDAR) system for determininga distance to and velocity of a target by scanning a beam through a 360°field-of-view (FOV) and processing beams reflected off of the target,said system comprising: means for generating a laser beam to betransmitted; means for frequency modulating the laser beam over afrequency modulation range; means for directing the frequency modulatedlaser beam to a spiral phase plate resonator (SPPR) device; means fordirecting a transmitted beam from the SPPR device onto a conical mirrorthat directs the transmitted beam at a certain angle therefrom dependingon a frequency of the laser beam; means for receiving a reflected beamfrom the target; and means for processing the reflected beam.